Cross-point memory and methods for fabrication of same

ABSTRACT

The disclosed technology generally relates to integrated circuit devices, and in particular to cross-point memory arrays and methods for fabricating the same. Line stacks are formed, including a storage material line disposed over lower a conductive line. Upper conductive lines are formed over and crossing the line stacks, exposing portions of the line stacks between adjacent upper conductive lines. After forming the upper conductive lines, storage elements are formed at intersections between the lower conductive lines and the upper conductive lines by removing storage materials from exposed portions of the line stacks, such that each storage element is laterally surrounded by spaces. A continuous sealing material laterally surrounds each of the storage elements.

BACKGROUND

1. Field

The disclosed technology generally relates to integrated circuitdevices, and in particular to cross-point memory arrays and methods forfabricating the same.

2. Description of the Related Art

Some memory devices include memory cells that can switch by changingtheir resistance state in response to an electrical signal, such as avoltage or a current pulse. Such memory cells, sometimes called variableresistance memory cells, include active elements that include variableresistance materials, whose electrical resistance can be changed by theelectrical signal. In some variable resistance change materials, thechange in electrical resistance can be accompanied by generation ofheat. One category of such variable resistance materials is phase changematerials, whose change in resistance can be associated with generationof heat during switching. The heat generated during switching of amemory cell can induce adverse effects on neighboring cells, such asthermal disturbance, whose effects become more pronounced as the memorycells are scaled in dimensions. Thus, there is a need to reduce theadverse effects of the heat on the neighboring memory cells in memorydevices that include variable resistance materials.

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out in the concludingportion of the specification. However, organization and/or method ofoperation, together with certain objects, features, and/or advantagesthereof, may be better understood by reference to the following detaileddescription if read with the accompanying drawings in which:

FIG. 1A is a schematic vertical cross section of a memory arrayincluding phase change materials, taken in a first direction parallel toa lower conductive line.

FIG. 1B is a schematic vertical cross section of the memory array ofFIG. 1A, taken in a second direction perpendicular to the firstdirection and parallel to an upper conductive line.

FIG. 1C is a schematic horizontal cross section of the memory array ofFIGS. 1A and 1B, taken in a third direction perpendicular to the firstand second directions and parallel to the underlying substrate surface.

FIG. 2A is a schematic vertical cross section of a memory arrayincluding phase change materials, taken in a first direction parallel toa lower conductive line, according to some embodiments.

FIG. 2B is a schematic vertical cross section of the memory array ofFIG. 2A, taken in a second direction perpendicular to the firstdirection and parallel to an upper conductive line, according to someembodiments.

FIG. 2C is a schematic horizontal cross section of the memory array ofFIGS. 2A and 2B, taken in a third direction perpendicular to the firstand second directions and parallel to the underlying substrate surface,according to some embodiments.

FIG. 2D is an enlarged view of one of the memory cell stacks of FIG. 2C.

FIG. 2E is an enlarged view of a memory cell stack similar to FIG. 2D inaccordance with an alternate embodiment.

FIG. 3 is a graph showing calculated temperatures of neighboring memorycells including phase change materials, as a function of the temperatureof a target memory cell, according to some embodiments.

FIGS. 4A, 4C, 4E, 4G, 41 and 4K are schematic vertical cross sections ofintermediate structures a memory array including phase change materialsat various stages of fabrication, taken in a first direction parallel toa lower conductive line, according to some embodiments.

FIGS. 4B, 4D, 4F, 4H, 4J and 4L are schematic vertical cross sections ofthe intermediate structures corresponding to FIGS. 4A, 4C, 4E, 4G, 41and 4K, respectively, taken in a second direction perpendicular to thefirst direction and parallel to an upper conductive line, according tosome embodiments.

FIGS. 5A, 5C and 5E are schematic vertical cross-sections ofintermediate structures a memory array including phase change materialsat various stages of fabrication, taken in a first direction parallel toa lower conductive line, according to some other embodiments.

FIGS. 5B, 5D and 5F are schematic vertical cross sections of theintermediate structures corresponding to FIGS. 5A, 5C and 5E,respectively, taken in a second direction perpendicular to the firstdirection and parallel to an upper conductive line, according to someother embodiments.

Features in the drawings are not necessarily drawn to scale and mayextend in different directions from that illustrated. While various axesand directions are illustrated to facilitate the discussion herein, itwill be appreciated that the features may extend in differentdirections.

DETAILED DESCRIPTION

Some memory devices include memory cells that can switch by changingtheir resistance state in response to an electrical signal, such as avoltage or a current pulse. Such memory cells, sometimes called variableresistance memory cells, include active elements that include variableresistance materials, whose electrical resistance can be changed by theelectrical signal. One category of variable resistance materials isphase change materials, whose change in resistance state is associatedwith changes in the phase of the phase change materials betweencrystalline and amorphous states. The change in phase of the phasechange materials can be accompanied by generation of heat. The heatgenerated during switching of a memory cell can induce adverse effectson neighboring cells, such as thermal disturbance, whose effects becomemore pronounced as the memory cells are scaled in dimensions.

In some memory devices, the variable resistance memory cells arearranged in a cross-point array configuration. Generally, a cross-pointmemory array refers to a memory array having memory elements disposedand electrically connected at intersections between a first set ofconductive lines (e.g., word lines) and a second set of conductive lines(e.g., digit lines) overlapping and crossing the first set of conductivelines. The electrical resistance of the variable resistance materials ofthe cross-point memory array, e.g., phase change materials, can bechanged by the electrical signals provided through the first and secondconductive lines connected to the variable resistance materials.

FIGS. 1A-1C illustrate an example of a cross-point memory array 10having a substrate 18 and a plurality of memory cells 30 formed thereon,viewed in a y-direction, an x-direction, and a z-direction,respectively. Referring to FIGS. 1A and 1B, each of the memory cells 30is a variable resistance memory cell, e.g., a phase change memory cell,arranged in a stacked configuration between an upper conductive line 20extending in the y-direction and a lower conductive line 22 extending inthe x-direction. The upper and lower conductive lines 20 and 22 areconductors configured to carry electrical signals such as, for example,a voltage or a current pulse, between memory cells 30 and peripheralcircuitry such as driver circuitry and sensing circuitry (not shown).The memory cell 30 includes a first active element 38, e.g., a selectorelement, and a second active element 34, e.g., a storage element, and inthe illustrated embodiment these elements are separated by a middleelectrode 36. The illustrated memory cell 30 additionally includes alower electrode 40 between the first active element 38 and the lowerconductive line 22 and an upper electrode 32 between the upperconductive line 20 and the second active element 34.

Referring to FIG. 1A, opposing sidewalls (in the x-direction) of theupper conductive line 20 and first opposing sidewalls (in thex-direction) of the memory cell 30 are lined with a first sealingmaterial 46, and spaces between adjacent upper conductive lines 20 andspaces between adjacent memory cells 30 are filled with a firstisolation material 50. Referring to FIG. 1B, opposing sidewalls (in they-direction) of the lower conductive line 22 and second opposingsidewalls (in the y-direction) of the memory cell 30 are lined with asecond sealing material 52, and spaces between adjacent lower conductivelines 22 and spaces between adjacent memory cells 30 are filled with asecond isolation material 48. Referring to the horizontal cross sectionof FIG. 1C, the first sealing material 46 extends in the y-direction totraverse a plurality of memory cells 30 such that sidewalls of theplurality of memory cells 30 are lined and interconnected by acontinuous first sealing material 46. In contrast, the second sealingmaterial 52 extends in the x-direction to line sidewalls of individualmemory cells 30 but does not interconnect a plurality of memory cells30. The sealing materials 46 and 52 can function, during operationand/or processing, to minimize cross-contamination and/or materialinter-diffusion between various elements of the memory cell 30 andsurrounding materials, e.g., neighboring memory cells and isolationmaterials.

When the memory cells 30 are phase change memory cells, the temperaturesof the first active element 38 and/or the second active element 34 of anaccessed memory cell can be raised substantially above room temperature.The accessed memory cell, whose peak temperature can reach severalhundred degrees, can have detrimental effects on the neighboring cellsdue to thermal diffusion. For example, the temperatures of theneighboring memory cells can result in degradation in data retention anddisturbance. Sealing materials similar to the first sealing material 46(FIGS. 1A and 1C) that commonly envelope multiple memory cells 30 in adirection (e.g., y-direction) can thermally connect the cells along thatline, and result in a faster diffusion of heat in the direction comparedto a direction (e.g., x-direction) in which the memory cells 30 are notinterconnected by a common sealing material. Without being limited bytheory, it is believed that heat may more easily transmit alonginterfaces between the two different sealing materials 46 and 52.

In the following, various embodiments relating to memory devices andmethods of forming the memory devices are disclosed, which canadvantageously include continuous sealing materials surrounding eachmemory cell. The inventors have found the various embodiments tosubstantially reduce and/or equalize transference of heat from onememory cell to surrounding regions of the memory cell.

The term “continuous,” as used herein to describe materials such assealing layers or isolation materials, characterizes a material thatdoes not have gaps or interfaces formed within the material, such as maybe the case when a deposition process is performed in multiple steps, orinterrupted by an intervening process. In addition, the term “uniformmaterial,” as used herein to describe materials such as sealing layersor isolation materials, characterizes a material that does not havesubstantial disparities within the material, such as may be the casewhen a material, while connected, may be performed in multiple steps, orinterrupted by an intervening process. For example, a material that isformed in two separate deposition steps and/or intervened by anotherprocess step can be discontinuous and/or nonuniform if it hasidentifiable interfaces within the material, even if the material as awhole is formed of similar or same materials. For example, even if bothof the sealing materials 46 and 52 described above with respect to FIGS.1A-1C are formed of silicon dioxide materials, the resulting combinationof the sealing materials do not form a continuous and uniform sealingmaterial, due to at least one intervening process which creates seams atinterfaces formed by the sealing materials 46 and 52. Such seam may bedetected, for example, by staining with a dilute hydrofluoric acid, suchas that which may be used in the industry to enhance electron microscopyimages. Uniform material does not imply a uniform thickness.

FIGS. 2A, 2B and 2C are cross-sectional depictions of a cross-pointmemory array 100 having a plurality of memory cells 30, viewed in ay-direction, an x-direction, and a z-direction, respectively, accordingto some embodiments. Referring to FIGS. 2A and 2B, each of the memorycells 30 is a variable resistance memory cell, e.g., a phase changememory cell, arranged in a stacked configuration. In the illustratedembodiment, the memory cell 30 includes a relatively narrow stack whichforms a memory cell pillar formed at an intersection between a lowerconductive line 22 and an upper conductive line 20. The memory cell 30comprises a first active element 38 disposed over the lower conductiveline 22 and a second active element 34 disposed over the first activeelement 38. One of the first and second active elements 38 and 34comprises a storage material and the other of the first and secondactive materials comprises a selector material. In embodiments where thememory cells 30 is a phase change memory cell, at least one of the firstand the second active elements 38 and 34 can include a chalcogenidematerial that is configured to switch between crystalline and amorphousphases via an electrical pulse applied between one of the lowerconductive lines 22 and one of the upper conductive lines 20 during anaccess operation. The memory cell 30 can additionally include one ormore of a lower electrode 40 interposed between the lower conductiveline 22 and the first active element 38, a middle electrode 36interposed between the first active element 38 and the second activeelement 34, and an upper electrode 32 interposed between the upperconductive line 20 and the second active element 34. When included, oneor more of the electrodes 40, 36 and 32 can include a material, such ascarbon, that can serve simultaneously as an electrode material that canprovide relatively low contact resistance between adjacent materials, aswell as a diffusion barrier against intermixing, e.g., between metallines and chalcogenide elements, or between chalcogenide elements ofdifferent compositions. Thus, in the illustrated embodiment, a memorycell 30 forming a pillar includes a stack comprising the lower electrode40, the first active element 38, the middle electrode 36, the secondactive element 32 and the upper electrode 32. In other embodiments oneor both of the upper and lower electrodes can be patterned in linescoextensive with the adjacent upper or lower conductive line.

The memory array 100 further includes a continuous sealing material 56laterally surrounding each of the memory cell pillars of the memory cell30, which includes the first and second active elements 38 and 34 in theillustrated embodiment (FIGS. 2A, 2B and 2C). The memory array 100further includes initial sealing materials 52 formed on sidewalls ofeach of the memory cell pillars that are opposing each other in they-direction, as illustrated in FIGS. 2B and 2C. The initial sealingmaterials 52, however, are not formed on sidewalls of the memory cellpillars that are opposing each other in the x-direction and, as bestseem from FIG. 2C, the sealing material 56 further encloses the initialsealing materials 52 formed on the opposing sidewalls of the memory cellpillars in the y-direction. Unlike the first sealing material 46described above with respect to FIGS. 1A and 1C that interconnectmultiple memory cells 30 in a direction (e.g., y-direction), the sealingmaterial 56 of FIGS. 2A-2C does not interconnect multiple memory cells30 such that a disparity of heat diffusion in one direction (e.g.,y-direction) compared to another direction (e.g., x-direction) describedabove with respect to FIGS. 1A-1C may be much less pronounced, orpractically non-existent from the stand point of detrimentally affectingneighboring memory cells while accessing a target memory cell. Forexample, when the second active element 34 is the storage element of aphase change memory cell, the sealing material 56 laterally encloses thestorage element in both first and second directions such that during anaccess operation of a target storage element, the temperatures of anadjacent neighboring storage element in the x-direction aresubstantially the same as the temperature of an adjacent neighboringstorage element in the y-direction.

Furthermore, referring to FIG. 2A, the continuous sealing material 56 ofthe illustrated embodiment continuously covers top and side surfaces ofthe upper conductive lines 20 that are opposing in the x-direction,opposing sidewalls of the memory cell 30 in the x-direction, and topsurfaces of the lower conductive line 22 that are disposed betweenadjacent memory cells 30. In the illustrated embodiment, the sealingmaterial 56 is in contact with the side surfaces of the memory cell 30that are opposing in the x-direction.

Referring to FIG. 2B, the initial sealing material 52 continuouslycovers side surfaces of the upper conductive lines 20 and of the memorycells 30 that are opposed in the y-direction, and top surfaces of thesubstrate 18 that are disposed between adjacent lower conductive lines22. It will be understood that the term “substrate” as used hereinencompasses bulk semiconductor substrates as well as any integratedlayers formed thereon. In addition, the continuous sealing material 56continuously covers the initial sealing material 52, such that thecontinuous sealing material 56 does not directly contact the sidewallsof the memory cells 30 and of the lower conductive lines 22 that areopposed in the y-direction. In other embodiments, the initial sealingmaterial is omitted and the continuous sealing material continuouslycovers and contacts side surfaces of the upper conductive lines and ofthe memory cells that are opposing in the y-direction, and top surfacesof the substrate that are disposed between adjacent lower conductivelines.

Still referring to FIG. 2B, the continuous sealing material 56 is formeddirectly on the sealing material 52 over an entire surface of thesealing material 52. Other embodiments are possible such as, forexample, embodiments illustrated in FIGS. 5A-5F and described below,where the continuous sealing material 56 is formed directly on the firstsealing material 52 at an upper portion of the memory cell 30, while notformed directly on the sealing material 52 at a lower portion of thememory 30. In these embodiments, the second active material 34 at theupper portion of the memory cell 30 may be a storage material of a phasechange memory cell, which may be more susceptible to thermaldisturbance, while the first active material 38 at the lower portion ofthe memory cell 20 may be a selector material of the phase memory cellwhich may be less susceptible to thermal disturbance.

Referring to FIG. 2C, each initial sealing material 52 extends in thex-direction to line a sidewall of a single memory cell 30, similar tothe second sealing material 52 described above with respect to FIG. 1C.However, unlike the first sealing material 46 of FIG. 1C, the continuoussealing material 56 does not extend in the y-direction to contact aplurality of memory cells 30. Instead, each continuous sealing material56 surrounds an individual memory cell 30, and in the illustratedembodiment, the sealing material 56 contacts sidewalls of the memorycells 30 that are opposing in the x-direction, while not directlycontacting sidewalls of the memory cells 30 that are opposing in they-direction due to the presence of the initial sealing material 52interposed between the sidewalls of the memory cells 30 and thecontinuous sealing material 56. In embodiments where the initial sealingmaterials 52 are omitted, each continuous sealing material 56 surroundsindividual memory cell 30 to contacts all sidewalls of the memory cells30.

FIG. 2D is an enlarged view of a memory cell similar to an individualmemory cell 30 of FIG. 2C, according to some embodiments. The continuoussealing material 56 surrounds the individual memory cell, and thecontinuous sealing material 56 contacts first sidewalls 58 a and 58 b ofthe individual memory cell that are opposing in the x-direction, whilenot directly contacting sidewalls 60 a and 60 b of the individual memorycell that are opposing in the y-direction. In the illustratedembodiment, portions 56′ of the continuous sealing material 56 on thefirst sidewalls 58 a and 58 b are thicker relative to portions 56″ ofthe continuous sealing material 56 over the second sidewalls 60 a and 60b. The different thicknesses between the portions 56′ and the portions56″ can result, for example, due to the presence of the upper conductivelines 20, resulting in a “shadowing effect,” when the continuous sealingmaterial 56 is formed. This effect can be more pronounced, for example,when the continuous sealing material 56 is formed, for example, usingdeposition techniques such as chemical vapor deposition (CVD), includingplasma enhanced CVD (PECVD), which can result in less conformal coveragecompared to techniques such as, for example, atomic layer deposition(ALD).

FIG. 2E is an enlarged view of a memory cell similar to the memory cellof FIG. 2D in accordance with an alternative embodiment. Similar to thememory cell 30 of FIG. 2D, the continuous sealing material 56 surroundsthe individual memory cell. In the illustrated embodiment, unlike thememory cell of FIG. 2D, however, the initial sealing material 52 isomitted. The continuous sealing material 56 contacts first sidewalls 58a and 58 b of the individual memory cell that are opposing in thex-direction, and further contacts second sidewalls 60 a and 60 b of theindividual memory cell that are opposing in the y-direction.Furthermore, in the illustrated embodiment, the continuous sealingmaterial 56 has a uniform thickness on the first sidewalls 58 a and 58 bas well as on the second sidewalls 60 a and 60 b, such that thecontinuous sealing material 56 is also uniformly thick on all sidewallsof the memory cell. In the illustrated embodiment, even though the upperconductive lines 20 are present when the continuous sealing material 56is formed, the continuous sealing material 56 can still be uniform inthickness when, for example, the continuous sealing material 56 isformed using deposition techniques such as ALD, which employsself-limiting or saturated deposition in each cycle and can result inmore conformal coverage compared to techniques such as, for example,CVD, despite the shadowing effect.

Thus, it will be appreciated that unlike FIGS. 1A-1C, the sealingmaterials 52 and 56 of FIGS. 2A-2C, in addition to functioning tominimize cross-contamination and/or material inter-diffusion amongvarious elements of the memory cell 30 and surrounding materials asdescribed above with respect to FIGS. 1A-1C, also function to reduce orminimize disparity in heat transfer from a target cell that is beingaccessed to neighboring memory cells.

In some embodiments, at least one of the initial sealing material 52 andthe continuous sealing material 56 can include a suitable dielectricmaterial adapted to protect various portions of the sidewalls of thememory cell 30 during processing while minimizing thermal communicationbetween adjacent memory cells 30. The initial sealing material 52 andthe sealing material 56 can include oxide or nitride materials, such assilicon oxide (e.g., SiO₂), aluminum oxide (e.g., Al₂O₃) and siliconnitride (e.g., Si₃N₄), among others.

In addition, in some embodiments, one or both of the initial sealingmaterial 52 and the continuous sealing material 56 can be advantageouslyselected to have thicknesses in a range between about 1 nm and about 10nm, between about 2 nm and about 8 nm, or between about 3 nm and about 7nm, for example about 5 nm. In other embodiments, one or both of theinitial liner sealing material 52 and the sealing material 56 canadvantageously selected to have thicknesses selected be in a rangebetween about 5% and about 25%, or between about 10% and about 20%, ofthe width of the spaces formed between adjacent memory cells 30.

Still referring to FIGS. 2A-2C, the memory array 100 further comprises acontinuous isolation material 54 surrounding the continuous sealingmaterial 56, according to some embodiments. In some embodiments, theisolation material 54 may comprise a dielectric material different thanthe initial sealing material and the continuous sealing material. Theisolation material 54 can include oxide or nitride materials, such assilicon oxide (e.g., SiO₂), aluminum oxide (e.g., Al₂O₃) and siliconnitride (e.g., Si₃N₄), among others. It will be appreciated that, incontrast to FIGS. 1A-1C, adjacent memory cells 30 in FIGS. 2A-2C areinterposed in both x and y directions by the isolation material 54 thatcomprises a single continuous material.

Various elements of the memory cell 30 are now described in more detail,according to various embodiments. Referring to FIGS. 2A-2C, upper and/orlower conductive lines 20 and 22 can comprise a metal. Examples of themetal include elemental metals such as Al, Cu, Ni, Cr, Co, Ru, Rh, Pd,Ag, Pt, Au, Ir, Ta, and W; conductive metal nitrides such as TiN, TaN,WN, and TaCN; conductive metal silicides such as tantalum silicides,tungsten silicides, nickel silicides, cobalt silicides and titaniumsilicides; and conductive metal oxides such as RuO₂.

Still referring to FIGS. 2A-2C, examples the second active element 34,which can be a storage element, include a chalcogenide-based phasechange storage element, a resistive random access memory (RRAM) storageelement (e.g., NiO, HfO₂, ZrO₂, Cu₂O, TaO₂, Ta₂O₅, TiO₂, SiO₂, Al₂O₃), aconductive bridge random access memory (CBRAM) storage element (e.g.,metal-doped chalcogenide), and/or a spin transfer torque random accessmemory (STT-RAM) storage element, among other types of storage elements.

Examples of the first active element 38, which can be a selectorelement, include a two terminal device (e.g., a switch), such as adiode, an ovonic threshold switch (OTS), a tunnel junction, or a mixedionic electronic conduction switch (MIEC), among other two terminaldevices.

In embodiments where the memory cell 30 is a phase change memory cell,one or both of the first and second active elements 38 and 34, which canbe selector and storage elements, respectively, can comprisechalcogenide materials. When both storage and selector elements comprisechalcogenide materials, the storage element can comprise a chalcogenidematerial that can undergo a phase change that is nonvolatile at roomtemperature. On the other hand, the selector element can comprise achalcogenide material that does not undergo a similar nonvolatile phasechange.

In some embodiments, the storage element includes a chalcogenidematerial such as an alloy including at least two of the elements withinthe indium(In)-antimony(Sb)-tellurium(Te) (IST) alloy system, e.g.,In₂Sb₂Te₅, In₁Sb₂Te₄, In₁Sb₄Te₇, etc., or an alloy including at leasttwo of the elements within the germanium(Ge)-antimony(Sb)-tellurium(Te)(GST) alloy system, e.g., Ge₈Sb₅Te₈, Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, Ge₁Sb₄Te₇,Ge₄Sb₄Te₇, etc., among other chalcogenide alloy systems.

In some embodiments, the selector element includes a chalcogenidematerial. A selector device having a chalcogenide material can sometimesbe referred to as an Ovonic Threshold Switch (OTS). An OTS may include achalcogenide composition including any one of the chalcogenide alloysystems described above for the storage element and, in addition,further include an element that can suppress crystallization, such asarsenic (As), nitrogen (N) and carbon (C), to name a few. Examples ofOTS materials include Te—As—Ge—Si, Ge—Te—Pb, Ge—Se—Te, Al—As—Te,Se—As—Ge—Si, Se—As—Ge—C, Se—Te—Ge—Si, Ge—Sb—Te—Se, Ge—Bi—Te—Se,Ge—As—Sb—Se, Ge—As—Bi—Te, and Ge—As—Bi—Se, among others.

Still referring to FIGS. 2A-2C, the upper, middle and lower electrodes32, 36 and 40 can comprise materials that electrically connect theoperational elements of the memory cell but prevent interactions and/orinterdiffusion among adjacent materials. For example, depending upon theadjacent materials, suitable electrode materials can include one or moreconductive and semiconductive materials such as, for example, carbon(C); n-doped polysilicon and p-doped polysilicon; metals including, Al,Cu, Ni, Cr, Co, Ru, Rh, Pd, Ag, Pt, Au, Jr, Ta, and W; conductive metalnitrides including TiN, TaN, WN, and TaCN; conductive metal silicidesincluding tantalum silicides, tungsten silicides, nickel silicides,cobalt silicides and titanium silicides; and conductive metal oxidesincluding RuO₂.

Still referring to FIGS. 2A-2C, in some embodiments, the upper and lowerconductive lines 20 and 22 can have a lateral dimensions, e.g., widths,in x and y directions, respectively, selected to be in the range betweenabout 5 nm and 60 nm, for example about 30 nm, in the range betweenabout 5 nm and 40 nm, for example about 25 nm, or between about 5 nm and30 nm, for example about 20 nm, depending upon the lithographic node forthe integrated circuit design. Smaller dimensions are yet possible,limited only by the lithographic capability employed by the personskilled in the art. The upper and lower conductive lines 20 and 22 canhave lengths in the y and x directions, respectively, selected to bemuch greater than the widths, such as at least 100 times greater or atleast 1000 times greater than the widths.

In addition, in some embodiments, each element in the memory cell 30,including the first and second active elements 38 and 34, have lateraldimensions in both x and y dimensions that are each selected to be inthe range between about 5 nm and 60 nm, for example about 30 nm, in therange between about 5 nm and 40 nm, for example about 25 nm, or betweenabout 5 nm and 30 nm, for example about 20 nm, depending upon thelithographic node for the integrated circuit design.

FIG. 3 is a graph 120 showing calculated temperatures of neighboringmemory cells as a function of the temperature of a memory cell during anaccess operation, according to some embodiments. The calculationcorresponds to that of storage elements of phase change memory cells,similar to those described above with respect to FIGS. 1A-1C and FIGS.2A-2C. As used herein, the memory cell being accessed may be called a“target cell.” The x-axis represents the temperature of the storageelement of the target memory cell, similar to the memory cell 30described with respect to FIGS. 1A-1C and 2A-2C, that is being accessedin a programming operation, e.g., a SET operation or a RESET operation.As used herein, a SET operation refers to an operation which a targetmemory cell switches from a higher resistance state to a lowerresistance state, which may be accompanied by a phase change of thestorage element of the memory cell from a relatively amorphous state toa relatively more crystalline state. On the other hand, a RESEToperation refers to an operation which the target memory cell switchesfrom a lower resistance state to a higher resistance state, which may beaccompanied by a phase change of the storage element of the memory cellfrom a relatively crystalline state to a relatively more amorphousstate. As used herein, a SET operation refers to an access operation inwhich the target memory cell undergoes a change in resistance from arelatively high resistance state, e.g., RESET, state to a relatively lowresistance state, e.g., SET state. A RESET operation refers to an accessoperation in which the target memory cell undergoes a change inresistance from a relatively low resistance state, e.g., SET state, to arelatively high resistance state, e.g., RESET state. The y-axisrepresents the temperature of storage elements of nearest neighboringmemory cells, which can be memory cells adjacent the target cell in x(e.g., word line) or y (e.g., digit line) directions. The neighboringmemory cell, whose temperature can increase as a result of thetemperature of the target cell, can sometimes be called a “victim cell.”The calculated temperatures of the storage elements of the neighboringmemory cell in FIG. 3 represent steady-state temperatures.

Still referring to FIG. 3, the graph 120 shows calculated temperaturecurves 124 and 122 corresponding to storage elements of nearestneighboring memory cells in the y-direction (e.g., digit line direction)and in the x-direction (e.g., word line direction), respectively, for amemory array similar to the memory array 10 of FIGS. 1A-1C, in which afirst sealing material 46 extends in the y-direction to interconnectsidewalls of a plurality of memory cells 30. As illustrated, the nextnearest neighboring memory cell in the y-direction has a highertemperature compared to the next nearest neighboring memory cell in thex-direction, and the difference in temperature between the next nearestneighboring cells in the y and x directions becomes larger at highertemperatures of the target cell.

Still referring to FIG. 3, the graph 120 shows calculated temperaturecurves 126 and 128 corresponding to nearest neighboring memory cells inthe y-direction (e.g., digit line direction) and the x-direction (e.g.,word line direction), respectively, for a memory array similar to thememory array 100 of FIGS. 2A-2C, which includes a continuous sealingmaterial 56 comprising a single layer that surrounds at least the upperportions of the memory cells that include storage elements. Asillustrated, the calculated temperature curves 126 and 128 substantiallycoincide with each other throughout a temperature range between about300° C. and about 1200° C., such that a nearest neighboring memory cellsy- and x-directions have substantially the same temperatures, and thatthe difference in temperature between the next nearest neighboring cellsin the x and y directions is less than about 10 degrees within thetemperature range plotted. That is, the sealing material similar to thesealing material 56 of FIGS. 2A-2C does not interconnect multiple memorycells such that a disparity of heat diffusion in one direction comparedto another direction is much less pronounced than that between thecalculated temperature curves 122 and 124, or practically non-existent(e.g., within about 10° C.), from the stand point of detrimentallyaffecting neighboring memory cells while accessing a target memory cell.Moreover, the calculate temperature curves 126 and 128 are much lowerthan the curve 124, illustrating that thermal dispersion is retarded bythe continuously surrounding sealing material 56 relative to thecombination of sealing materials 52 and 46 in FIG. 1C, with itsattendant interfaces.

In the following, methods of fabricating cross-point memory arrayssimilar to the cross-point memory array 100 of FIGS. 2A-2C according tovarious embodiments are described, wherein the memory arrays havecontinuous sealing materials surrounding at least storage elementsformed at upper portions of the memory cell pillars. FIGS. 4A-4Lillustrate cross-sectional views of a cross-point memory array atvarious stages in a sequence of fabrication, according to someembodiments.

As used herein and throughout the specification, “subtractivepatterning” refers to a process sequence where structures to be definedare patterned by the removal of material. For example, a “subtractivepatterning process” may include blanket provision of a material to bepatterned, followed by lithographically providing etch mask structuresoverlapping areas to be patterned, followed by etching through the mask,such that materials in areas covered by the mask structures areprotected while materials in exposed areas are removed by the etchremoval process. In contrast to subtractive patterning, damascenepatterning involves patterning voids in a first layer, blanketdepositing material into the voids and over the remaining parts of thefirst layer, and polishing away the overburden from over the first layerto leave the material of interest patterned in the voids.

In the following, it will be understood that while only short segmentsof certain elongated structures such as segments of upper conductivelines and lower conductive lines may be illustrated, in practice suchelongated structures may be much longer. In addition, while only a fewparallel conductive lines and memory cells may be illustrated, inpractice many parallel lines and memory cells may be formed to span amemory array.

In the following, FIGS. 4A, 4C, 4E, 4G, 41 and 4K representcross-sectional views of intermediate array structures of a cross-pointmemory array at various stages of fabrication, viewed in a y-direction(e.g., a digit line direction), and FIGS. 4B, 4D, 4F, 4H, 4J and 4Lrepresent cross-sectional views of the intermediate array structurescorresponding to the intermediate structures of FIGS. 4A, 4C, 4E, 4G, 41and 4K, respectively, viewed in an x-direction (e.g., a word linedirection).

Referring to intermediate array structure 100 a of FIGS. 4A and 4B, themethod of fabricating a memory array includes forming a memory cellmaterial stack on a substrate 18. The illustrated stack includes a lowerconductive material 22 a formed over the substrate 18, and the memorycell material stack is formed thereover, including a lower electrodematerial 40 a on the lower conductive material 22 a, a first activematerial 38 a (e.g., a selector element material) on the lower electrodematerial 40 a, a middle electrode material 36 a on the selector elementmaterial 38 a, a second active material 34 a (e.g., a storage elementmaterial) on the middle electrode material 36 a, and an upper electrodematerial 32 a on the storage element material 34 a. The foregoingfeatures of the lower conductive material 22 a and the memory cellmaterial stack may be formed, for example, by deposition techniques suchas physical deposition (PVD), chemical vapor deposition (CVD), andatomic layer deposition (ALD), among other deposition techniques. Eachfeature of FIGS. 4A and 4B may be formed as a blanket layer over anentire substrate 18, such as a wafer. Because the stack is unpatterned,it looks identical in both the y-direction view of FIG. 4A and thex-direction view of FIG. 4B.

It will be appreciated that at least one of features of the memory cellmaterial stack of the intermediate structure 100 a can be omitted andstill have functional memory cells upon completion of fabrication. Forexample, one of the first or second active materials 38 a or 34 a can beomitted in some embodiments, where the omitted active material is aselector material. In addition, one or more of the lower electrodematerial 40 a, the middle electrode material 36 a and the upperelectrode material 32 a can be omitted in some embodiments.

Referring to intermediate array structure 100 b of FIGS. 4C and 4D, themethod of fabricating the memory array additionally includessubtractively patterning the memory cell material stack and the lowerconductive material 22 a (FIGS. 4A and 4B) to form a memory cell linestack on a lower conductive line 22, both extending in the x-direction.In some embodiments, the memory cell line stack and the lower conductiveline 22 are patterned using a first photo mask and a first single etchprocess. The memory cell line stack includes a lower electrode line 40 bon the lower conductive line 22, a first active element line 38 b (e.g.,a storage element line) on the lower electrode line 40 b, a middleelectrode line 36 b on the first active element line 38 b, a secondactive element line 34 b (e.g., a storage element line) on the middleelectrode line 36 b, and an upper electrode line 32 b on the secondactive element line 34 b.

Still referring to intermediate array structure 100 b of FIGS. 4C and 4Dthe method additionally includes, after forming the memory cell linestacks, forming initial sealing material 52 a on sidewalls of the linestacks extending in the x-direction, as illustrated in FIG. 4D. Theinitial sealing material 52 a can include suitable dielectric materialssuch as, for example, silicon oxide and silicon nitride, which may bedeposited by a suitable deposition process such as chemical vapordeposition (CVD) and atomic layer deposition (ALD). CVD and ALD candeposit the initial sealing material 52 a conformally, with ALDproviding better conformality than CVD. Plasma-enhanced CVD (PECVD) canalso provide relatively conformal deposition at low temperaturescompatible with sensitive phase change materials like chalcogenidematerials, which can migrate during processing at higher temperatures.

Still referring to intermediate array structure 100 b of FIGS. 4C and4D, inter-line spaces between adjacent memory cell line stacks arefilled with a dielectric material to form initial isolation dielectricregions 48 a. Suitable dielectric materials to fill the spaces caninclude, for example, silicon oxide and silicon nitride, which may bedeposited by a suitable gap-filling process known in the art. In someembodiments, the dielectric materials of the initial isolationdielectric regions 48 a are different from the dielectric material ofthe initial sealing material 52 a. Once the inter-line spaces betweenadjacent memory cell line stacks are filled, the intermediate arraystructure 100 b can be chemical-mechanically polished to stop on theupper electrode lines 32 b, thus exposing a substantially planar surface(not shown) comprising alternating surfaces of the upper electrode lines32 b interposed by isolation dielectric regions 48 a. Thus, in theillustrated embodiment, chemical-mechanically polishing forms the memorycell line stacks and initial isolation dielectric regions 48 a thatalternate in the y-direction.

Referring intermediate array structure 100 c of FIGS. 4E and 4F, themethod of fabricating the memory array additionally includes depositingan upper conductive material (not shown) on the substantially planarsurface and subtractively patterning using a second photo mask to form aplurality of upper conductive lines 20 extending in the y-direction. Theupper conductive material can comprise similar or same material as thelower conductive line 22 and can be formed using substantially similaror same processes as discussed above for forming the lower conductiveline 22. Thus formed, the plurality of upper conductive lines 20 aredisposed on the alternating memory cell line stacks and the initialisolation dielectric regions 48 b, wherein the upper conductive linesextend in the y-direction to cross the memory cell line stacks thatextend in the x-direction. Forming the upper conductive lines 20 exposesportions of the upper electrode lines 32 b of the alternating linestacks, as best seen from FIG. 4E, and portions of the initial isolationdielectric regions 48 b between adjacent upper conductive lines 20. Theupper conductive lines 20 intersecting the line stack form anoverlapping region between the line stack and the upper conductive line.

Referring to intermediate array structure 100 d of FIGS. 4G and 4H, themethod of fabricating the memory array additionally includes recessingto remove at least upper portions of the initial isolation dielectricmaterials from exposed portions of the initial isolation dielectricregions 48 b to form spaces 48 c. In the illustrated embodiment, entireinitial isolation dielectric materials are removed from the initialdielectric regions 48 b to form the spaces 48 c. The initial isolationdielectric materials can be removed using, for example, wet or dryetching techniques that selectively remove the isolation dielectricmaterials while substantially leaving intact remaining structures,including the initial sealing material 52 b. For example, in embodimentswhere the isolation dielectric regions 48 b are formed of silicondioxide and the initial sealing material 52 b is formed of siliconnitride, an etchant including dilute hydrofluoric acid (HF) canselectively remove the isolation dielectric materials to form the spaces48 c.

Referring to intermediate structure 100 e of FIGS. 41 and 4J, afterremoving the initial isolation dielectric materials from the isolationdielectric regions 48 b to form the spaces 48 c, the method of formingthe memory array further includes removing at least upper portions ofthe exposed portions of the memory cell line stacks of FIGS. 4G and 4Hto form free-standing pillars at intersections of the lower conductivelines 22 and the upper conductive lines 20. In the illustratedembodiment of FIGS. 41 and 4J, entire exposed portions (upper and lowerportions) of the memory cell line stacks are removed by stopping theetch at the lower conductive lines 22 (or at an etch stop layerthereover) such that the resulting free-standing pillar includes theupper electrode 32, the second active element 34 (e.g., a storageelement), the middle electrode 36, the first active element 38 (e.g., aselector element), and the lower electrode element 40. In otherembodiments, the etch can be stopped after etching any layer above thelower conductive lines 22 such that any one of the cell stack componentlayers, e.g., the lower electrode 40 or the first active element 38 canform a line similar to the lower electrode line 22. Thus, forming thefree-standing pillars at intersections between the lower conductivelines 22 and the upper conductive lines 20 by removing the second activematerials (e.g., storage materials) or both the second and first activematerials (e.g., selector materials) from exposed portions of the memorycell line stacks of FIGS. 4G and 4H include forming the free-standingpillars that are laterally surrounded by spaces 50.

It will be appreciated that, because the upper conductive lines 20 usingthe second photo mask are formed prior to forming the free-standingpillars and can act as an etch mask (together with, for example, hardmasks that may be on the upper conductive lines 20, not shown), formingthe free-standing pillar can be performed without an additionalphotolithography process. Thus, the same pattern employed for the upperconductive lines 20 can also be used for etching pillars out of thememory cell line stacks. Between upper conductive lines 20, the etchremoves exposed portions of the initial sealing material 52 b as well asthe materials of the memory cell line stacks.

Referring to intermediate array structure 100 of FIGS. 4K and 4L, afterforming the free standing pillars between the lower and upper conductivelines 22 and 20, the method of forming the memory array includes forminga continuous sealing material 56 laterally surrounding upper portions ofeach of the free-standing portions. In the illustrated embodiment, thesealing material 56 laterally surrounds the entire free standing pillars(upper and lower portions) of the intermediate array structure 100 e ofFIGS. 41 and 4J, which includes the upper electrode 32, the secondactive element 34 (e.g., a storage element), the middle electrode 36,the first active element 38 (e.g., a selector element), and the lowerelectrode element 40. However, as described above with respect to FIGS.41 and 4J, the sealing material 56 can be formed to surround a subset ofthese features, depending on where the etch process is stopped to formthe free-standing pillar of the intermediate array structure 100 e. Inthe illustrated embodiment, the sealing material 56 directly contactssidewalls of the memory cell 30 that oppose each other in thex-direction, as best seen from FIG. 4K, while contacting the initialsealing material 52 on sidewalls of the memory cell 30 that oppose eachother in the y-direction, as best seen from FIG. 4L. However, in someembodiments, the initial sealing material 52 can be omitted, such thatthe sealing material 56 contacts all sidewalls of the memory cell 30, asillustrated in FIG. 2E. After forming the sealing material 56, themethod of forming the memory array further comprises forming anisolation material 54 continuously surrounding the sealing material 56,wherein the isolation material 54 can be formed of a material similar tothe isolation regions 48 b of FIG. 4F. In some embodiments, theisolation material 54 comprises a dielectric material different than theinitial sealing material 52 and/or the continuous sealing material 56.In subsequent processes, the intermediate array structure 100 may bechemical-mechanically polished to remove the isolation material 54 abovethe top surfaces of the upper conductive lines 20 (not shown).

Thus, intermediate array structure 100, similar to the memory array 100of FIGS. 2A-2D, includes a continuous sealing material 56 laterallysurrounding each of the memory cell pillars of the memory cell array.The memory cell pillar includes the first and second active elements 38and 34 in the illustrated embodiment, and further includes initialsealing materials 52 formed on sidewalls of each of the memory cellpillars that are opposing each other in the y-direction. The continuoussealing material 56 has no interfaces through which heat within the cell30 can transfer relatively fast to and from adjacent cells.

FIGS. 5A-5F illustrate cross-sectional views of a memory array atvarious stages of fabrication according to alternative embodiments. Inthe following, FIGS. 5A, 5C and 5E represent cross-sectional views ofintermediate array structures of a cross-point memory array at variousstages of fabrication, viewed in a y-direction (e.g., a digit linedirection), and FIGS. 5B, 5D and 5F represent cross-sectional views ofthe intermediate array structures corresponding to the intermediatestructures of FIGS. 5A, 5C and 5E, respectively, viewed in anx-direction (e.g., a word line direction). Processes leading up to theintermediate structures of FIGS. 5A and 5B are similar to processesleading up to the intermediate structures of FIGS. 4E and 4F. Thus,prior to forming an intermediate array structure 120 a of FIGS. 5A and5B, forming the upper conductive lines 20 exposes portions of the upperelectrode lines 32 b of the alternating line stacks, as seen in FIG. 5A,and portions of the initial isolation dielectric regions betweenadjacent upper conductive lines 20, similar to the initial isolationdielectric regions 48 b described above with respect to FIGS. 4E and 4F.

Referring to FIGS. 5A and 5B, the intermediate array structure 120 a issimilar to the intermediate array structure 100 d of FIGS. 4G and 4H,except that removing at least upper portions of the initial isolationdielectric materials is a partial recess process that at least exposessidewalls of the storage element of upper portions the memory cell linestacks. In the illustrated embodiment, the exposed sidewalls are coveredwith the initial sealing material 52 b. In some other embodiments,however, the initial sealing material 52 b may be omitted. In theillustrated embodiment, lower portions of the initial isolationdielectric regions 48 d are left in place such that the upper portionsof the spaces 50 a extend to partially expose upper sidewalls of thememory cell line stacks while the sidewalls of lower portions of thememory cell line stacks remain covered by remaining isolation dielectricregions 48 d. In the illustrated embodiment, exposed upper portions ofmemory cell line stacks include the upper electrode line 32 b and thesecond active element 34 b (e.g., storage element). It will beappreciated, however, that in other embodiments, the exposed memory cellline stacks can further include features below the second active element34 b.

Referring to FIGS. 5C and 5D, intermediate array structure 120 b isformed by etching the memory cell line stacks using the upper conductivelines 20 (and any hard mask materials thereover) as a mask to formspaces 50 a around memory cell pillars. Intermediate array structure 120b is similar to the intermediate array structure 100 e of FIGS. 41 and4J, except that the remaining isolation dielectric regions 48 are leftbetween memory cell pillars underneath the upper conductive lines 20.Thus side walls of the memory cell pillars that are opposing each otherin the x-direction are exposed by the spaces 50 a (see FIG. 5C), whileside walls of the memory cell pillars that are opposing each other inthe y-direction are only partially exposed by the spaces 50 a (see FIG.5D). In some embodiments, at least the storage elements 34 of upperportions of the memory cell pillars have all of their sidewalls exposedand surrounded by the spaces 50 a. In the illustrated embodiment, thestorage element is represented by the second active element 34. In otherembodiments, where the storage element could be the first active element38, the recess in initial isolation material 48 can be deeper to exposeat least the storage element sidewalls. Lower portions of the pillarsremain covered by the remaining isolation dielectric regions 48.

Referring to FIGS. 5E and 5F, intermediate array structure 120 is formedby forming a continuous sealing material 66 around the exposed portionsof the memory cell pillars, followed by deposition of a filler materialin the form of an isolation material 64. The intermediate arraystructure 120 is similar to the intermediate array structure 100 ofFIGS. 4K and 4L, except that the continuous sealing material 66laterally surrounds only upper portions of the free-standing pillars ofFIGS. 5C and 5D, while not surrounding lower portions below the uppersurface of the remaining isolation dielectric regions 48. In theillustrated embodiment, the sealing material 66 laterally surrounds theupper portions of the memory cell pillars of the intermediate arraystructure 120 b of FIGS. 5C and 5D, which includes the upper electrode32 and the second active element 34 (e.g., a storage element). Themiddle electrode 36, the first active element 38 (e.g., a selectorelement), and the lower electrode element 40 are not surrounded by thesealing material 66 and are rather only lined on two opposing walls, asshown in FIG. 5E. After forming the sealing material 66, the method offorming the memory array further comprises forming an isolation material64 which fills the gaps among the memory cell pillars and the remainingisolation regions 48, including surrounding the continuous sealingmaterial 66 at upper portions of the pillars. The isolation material 64can be formed of a material similar to the isolation regions 48. Insubsequent processes (not shown), the intermediate array structure 120may be chemical-mechanically polished to remove the isolation material64 above the top surfaces of the upper conductive lines 20.

Although this invention has been described in terms of certainembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Moreover, the various embodiments described above can becombined to provide further embodiments. In addition, certain featuresshown in the context of one embodiment can be incorporated into otherembodiments as well. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

What is claimed is:
 1. A method of fabricating a memory array,comprising: forming a plurality of line stacks, wherein each line stackincludes a storage material line disposed over a lower conductive line;forming a plurality of upper conductive lines on and crossing the linestacks, wherein forming the upper conductive lines exposes portions ofthe line stacks between adjacent upper conductive lines; after formingthe upper conductive lines, forming a plurality of storage elements atintersections between the lower conductive lines and the upperconductive lines by removing storage materials from the storage materiallines such that each storage element is laterally surrounded by spaces;and forming a continuous sealing material laterally surrounding each ofthe storage elements.
 2. The method of claim 1, wherein: forming theline stacks comprises using a first photo mask to pattern a materialstack including a storage material disposed over a lower conductivematerial; forming the upper conductive lines comprises using a secondphoto mask to pattern an upper conductive material; and forming thestorage elements comprises etching without using an additional photomask.
 3. The method of claim 1, further comprising: after forming theplurality of line stacks, filling gaps between the line stacks with aninitial isolation material; and after forming the plurality of upperconductive lines and before forming the storage elements, removing atleast an upper portion of the initial isolation material to exposesidewalls of the storage material lines.
 4. The method of claim 3,wherein forming the plurality of line stacks further includes forming aplurality of selector material lines interposed between the storagematerial lines and the lower conductive lines.
 5. The method of claim 4,wherein removing the at least an upper portion of the initial isolationmaterial includes further removing lower portions of the initialisolation material to expose sidewalls of the selector material lines,further comprising after forming the upper conductive lines, forming aplurality of selector elements interposed between storage elements andthe lower conductive lines by removing selector materials from theselector material lines such that each selector element is laterallysurrounded by the spaces, and wherein forming the continuous sealingmaterial further comprises laterally surrounding each of the selectorelements.
 6. The method of claim 4, wherein removing the at least anupper portion of the initial isolation material includes leaving lowerportions of the initial isolation material such that sidewalls of theselector material lines remain unexposed.
 7. The method of claim 1,further comprising, after forming the sealing material, furthersurrounding the storage elements with an isolation material by fillingthe spaces laterally around the storage elements.
 8. The method of claim1, wherein forming the line stacks comprises forming and patterningstorage materials including phase change materials.
 9. The method ofclaim 1, further comprising, after forming the line stacks, forminginitial sealing materials on sidewalls of the line stacks prior toforming the storage elements.
 10. The method of claim 9, wherein formingthe storage elements includes removing the initial sealing materials onsidewalls of the storage material line of the exposed portions of theline stacks, and wherein forming the continuous sealing materialcomprises forming the sealing material on remaining initial sealingmaterials on first opposing sidewalls of the each of the storageelement, and forming the sealing material on second opposing sidewallsof the each of the storage elements.
 11. The method of claim 1, whereinforming the line stacks further comprises forming the each line stackincluding a selector material line interposed between the lowerconductive line and the storage material line.
 12. The method of claim11, further comprising, after forming the upper conductive lines,forming a plurality of selector elements, wherein each storage elementis vertically interposed between one of the storage elements and a lowerconductive line.
 13. The method of claim 12, wherein forming thecontinuous sealing material further comprises laterally surrounding eachof the selector elements.
 14. A method of fabricating a memory device,comprising: forming a line stack comprising a memory cell line stackdisposed on a lower conductive line; forming an upper conductive lineintersecting the line stack to form an overlapping region between theline stack and the upper conductive line; forming a free-standing pillarat the overlapping region by removing at least an upper portion of thememory cell line stack from non-overlapping regions adjacent theoverlapping region; and forming a continuous sealing material coveringlateral surfaces of the free standing pillar.
 15. The method of claim14, wherein forming the line stack is performed using a firstphotolithography mask, forming the upper conductive line is performedusing a second photolithography mask, and forming the free-standingpillar is performed without an additional photolithography process. 16.The method of claim 14, wherein forming the line stack includes forminga lower active element line of the memory cell stack over the lowerconductive line and an upper active element line over the lower activeelement line, and wherein one of the upper and lower active elementlines comprises a storage element line and the other of the upper andlower active materials comprises a selector element line.
 17. The methodof claim 16, wherein forming the free-standing pillar includes removingthe upper active element line from the non-overlapping regions of thecell line stack to form an upper active element.
 18. The method of claim17, wherein the upper active element comprises the storage element. 19.The method of claim 16, wherein forming the free-standing pillar furtherincludes removing the lower active element line from the non-overlappingregions of the cell line stack to form a lower active element.
 20. Themethod of claim 19, wherein forming the free-standing pillar includesforming the lower active element after forming the continuous sealingmaterial.
 21. A method of fabricating a memory array, comprising:forming a plurality of line stacks and initial isolation dielectricmaterials, wherein the line stacks and the initial isolation dielectricmaterials alternate in a lateral direction; forming a plurality of upperconductive lines on the alternating memory cell line stacks and theinitial isolation dielectric materials, the upper conductive linescrossing the line stacks, wherein forming the upper conductive linesexposes portions of the alternating line stacks and the initialisolation dielectric materials between adjacent upper conductive lines;removing upper portions of the initial isolation dielectric materials;removing upper portions of exposed portions of the line stacks to formfree-standing pillars at intersections of the lower conductive lines andthe upper conductive lines; and after removing the upper portions of theinitial isolation dielectric materials, lining to surround upperportions of the free-standing pillars with a continuous liner material.22. The method of claim 21, wherein each of the line stacks includes alower conductive line, a lower active element line over the lowerconductive line and an upper active element line over the lower activeelement line, and wherein forming the free-standing pillars includesforming upper active elements surrounded by spaces.
 23. The method ofclaim 22, further comprising: removing lower portions of the initialisolation dielectric materials; and removing lower portions of exposedportions of the line stacks to form the free-standing pillars, whereinforming the free-standing pillars further includes removing lowerportions of the line stacks to form lower active elements surrounded byspaces.
 24. The method of claim 23, wherein the upper active elementsare storage elements and the lower active elements are selectorelements.
 25. The method of claim 22, further comprising: after lining,removing lower portions of the line stacks to form lower activeelements.
 26. A memory device, comprising a plurality of lowerconductive lines extending in a first direction; a plurality of upperconductive lines extending in a second direction crossing the firstdirection; a plurality of memory cell pillars formed at intersectionsbetween the lower conductive lines and the upper conductive lines,wherein each of the memory pillars comprise a storage material elementlaterally surrounded by a continuous sealing material, wherein thecontinuous sealing material extends to line opposing sidewalls of theupper conductive line connected to the each of the memory pillars. 27.The memory device of claim 26, wherein the storage element of the eachof the memory pillars comprises a chalcogenide material configured toswitch between crystalline and amorphous phases during an accessoperation via an electrical pulse applied between one of the lowerconductive lines and one of the upper conductive lines.
 28. The memorydevice of claim 27, wherein the sealing material laterally encloses thestorage element in both first and second directions such that during anaccess operation to access a target storage element, the temperature ofan adjacent neighboring storage element in the first direction issubstantially the same as the temperature of an adjacent neighboringstorage element in the second direction.
 29. The memory array of claim28, wherein the sealing material further encloses an initial sealingmaterial formed on opposing first sidewalls of each of the storageelements, the first sidewalls extending in the first direction.
 30. Thememory array of claim 28, wherein the sealing material contacts opposingsecond sidewalls of each of the storage elements, the second sidewallsextending in the second direction.
 31. A memory device, comprising alower conductive line extending in a first lateral direction; an upperconductive line extending in a second lateral direction crossing thefirst lateral direction; a memory cell stack formed at an intersectionbetween the lower and upper conductive lines, the memory cell stackcomprising: a first active element disposed over the lower conductiveline, a second active element disposed over the first active element,wherein one of the first and second active elements comprises a storagematerial and the other of the first and second active materialscomprises a selector material; and a continuous sealing materiallaterally surrounding the second active element and extending to coversidewalls of the upper conductive line.
 32. The memory device of claim31 further comprising an isolation material continuously surrounding thecontinuous sealing material, wherein the isolation material comprises adielectric material different from the continuous sealing material. 33.The memory device of claim 31, wherein the continuous sealing materialcomprises silicon nitride.
 34. The memory device of claim 31, whereinthe continuous sealing material further laterally surrounds the firstactive element.
 35. The memory device of claim 31, wherein the secondactive element comprises the storage element and the first activeelement comprises the selector element, wherein the storage elementcomprises a first chalcogenide material.
 36. The memory device of claim35, wherein the selector element comprises a second chalcogenidematerial different from the first chalcogenide material.
 37. The memorydevice of claim 31, wherein the continuous sealing material contactsopposing sidewalls of second active element extending in the seconddirection.
 38. The memory device of claim 37, further comprising anisolation material surrounding the continuous sealing material.